This invention relates generally to planar optical devices and materials and methods used in their manufacture, and, in particular, to optical components such as waveguides and amplifiers, and physical vapor deposition methods for their manufacture.
The increasing prevalence of fiber optic communications systems has created an unprecedented demand for devices for processing optical signals. Planar devices such as optical waveguides, couplers, splitters, and amplifiers, fabricated on planar substrates, like those commonly used for integrated circuits, and configured to receive and process signals from optical fibers are highly desirable. Such devices hold promise for integrated optical and electronic signal processing on a single semiconductor-like substrate.
The basic design of planar optical waveguides and amplifiers is well known, as described, for example in U. S. Pat. Nos. 5,119,460 to Bruce et al., 5,613,995 to Bhandarkar et al., (hereafter ""995), 5,900,057 to Buchal et al., and 5,107,538 to Benton et al, to cite only a few. The devices consist, very generally, of a core region, typically bar shaped, of a certain refractive index surrounded by a cladding region of a lower refractive index. In the case of an optical amplifier, the core region contains a certain concentration of a dopant, typically a rare earth ion such as an erbium or praseodymium ion which, when pumped by a laser, fluoresces, for example, in the 1550 nm and 1300 nm wavelength range, respectively, used for optical communication, amplifying the optical signal passing through the core.
The performance of these planar optical devices depends sensitively on the value and uniformity of the refractive index of the core region and of the cladding region, and particularly on the difference in refractive index, xcex94n, between the regions. Particularly for passive devices such as waveguides, couplers, and splitters, xcex94n needs to be sensitively controlled at values less than 1% and the refractive index of both core and cladding need to be highly uniform, for some applications at the fewer than parts per thousand level. In the case of doped materials forming the core region of planar optical amplifiers, it is important that the dopant be uniformly distributed so as to avoid non-radiative quenching or radiative quenching, for example by upconversion. The refractive index and other desirable properties of the core and cladding regions, such as physical and chemical uniformity, low stress, and high density, depend, of course, on the choice of materials for the devices and on the processes by which they are fabricated.
Because of their optical properties, silica and refractory oxides such as Al2O3, are good candidate materials for planar optical devices. Further, these oxides serve as suitable hosts for rare earth dopants used in optical amplifiers. A common material choice is so-called low temperature glasses, doped with alkali metals, boron, or phosphorous, which have the advantage of requiring lower processing temperatures. In addition, dopants are used to modify the refractive index. Methods such as flame hydrolysis, ion exchange for introducing alkali ions in glasses, sputtering, and various chemical vapor deposition processes (CVD) have been used to form films of doped glasses. However, dopants, such as phosphorous and boron which are hygroscopic, and alkalis are undesirable for integration with electronic devices. Control of uniformity of doping in CVD processes can be difficult and CVD deposited films can have structural defects leading to scattering losses when used to guide light. In addition, doped low temperature glasses may require further processing after deposition. A method for eliminating bubbles in thin films of sodium-boro-silicate glass by high temperature sintering is described, for example, in the ""995 patent to Bhandarkar et al.
In the case of pure SiO2, the most uniform optical material presently known is by atmospheric pressure thermal oxide (APOX). The APOX process can provide a 13 xcexcm thick silica film having a precise refractive index of 1.4584, at 1550 nm, with a 1"sgr" variance in the refractive index across a 150 mm wafer of 3xc3x9710xe2x88x925. However, the APOX process does not provide a method of making films with different indices of refraction. It is, therefore, not suitable for forming a waveguide core film with a desired refractive index (n).
Thus, there remains a need for a process to provide optical materials with a specified and uniform index of refraction for planar optical devices. It would be desirable if the material additionally exhibits high optical transparency, low stress, and high density and is free of structural defects.
A physical vapor deposition process provides optical materials with controlled and uniform refractive index that meet the requirements for active and passive planar optical devices. According to a first aspect of the present invention, radio frequency (RF) sputtering of a wide area target in the presence of a sputtering gas under a condition of uniform target erosion is used to deposit physically and chemically uniform material on a substrate. The substrate is positioned opposite a planar target of the material to be deposited, the area of the target being larger than the area of the substrate. A central area of the target of the same size as the substrate and overlying the substrate is exposed to a uniform plasma condition, which provides a condition of uniform target erosion. A uniform plasma condition can be created without magnetic enhancement, termed diode sputtering, or by providing a time-averaged uniform magnetic field by scanning a magnet across the target in a plane parallel to the plane of the target.
According to an aspect of the present invention, a film deposited on the substrate using a wide area target and uniform target erosion is of uniform thickness for targets with an area at least 1.5 times the area of the substrate. In addition, film deposited on a substrate positioned opposite a central region of the target inside the region providing film thickness uniformity exhibits physical and chemical uniformity useful for fabricating optical devices. The region providing chemical uniformity can be coextensive with the region providing thickness uniformity.
According to another aspect of the present invention, a dual frequency RF sputtering process is used in which the high frequency RF power applied to the target is augmented by applying low frequency RF power to the target, resulting in densification of the deposited film and better coverage of features when deposited over underlying layers. Further, the dual frequency RF process can be used to tune the refractive index of the deposited film. Keeping the total RF power the same, the refractive index tends to increase with the ratio of low frequency to high frequency RF power.
In yet another method, RF power is applied to the substrate resulting in substrate bias. Substrate bias is used with single frequency or with dual frequency RF sputtering to provide improved density and morphology of deposited films and to complete coverage and filling of features on underlying layers. Furthermore, substrate bias contributes to uniformity of refractive index. Films deposited by diode sputtering including application of substrate bias demonstrate exceptional refractive index uniformity and low average surface roughness.
According to another aspect of the present invention, the refractive index of the material deposited using an RF sputtering process can be deliberately tuned by modifying other plasma processing conditions. First, raising the deposition temperature increases the refractive index of the resulting material. Second, increasing the RF power applied to the target increases the refractive index of the deposited material. Third, a reactive process gas can be added to the sputtering chamber which effectively modifies the chemical composition of the deposited material with a corresponding change in refractive index. Additionally, the refractive index of deposited material can be modified by using a target material in a specific oxidation state. The RF sputtering method is applicable to depositing pure materials and mixed materials including materials containing rare earth dopants for optical amplifier applications. Thus, wide area RF sputtering can be used together with the present refractive index control methods to provide core and cladding materials with a desired difference in refractive index for planar optical waveguides and optical amplifiers.
The present RF sputtering methods for material deposition and refractive index control are combined with processes commonly used in semiconductor fabrication to produce planar optical devices. A surface ridge optical device is produced by using RF sputtering to deposit a stack comprising an upper cladding layer, a middle core layer, and a lower cladding layer on a substrate. A ridge is etched into the upper cladding layer and partway through the thickness of the core layer to produce the surface ridge device. A buried ridge device is produced by etching a ridge into a layer of core material overlying a cladding layer. A top layer of cladding material is deposited over the core ridge by RF sputtering with substrate bias. Use of substrate bias enables the cladding layer to completely cover the exposed ridge without defects. Further, the deposition methods described here are used to fabricate a buried trench device in which RF sputtering with substrate bias completely fills a trench in a layer of cladding material with core material.
Finally, a method for forming composite wide area targets from multiple tiles is provided. The method includes positioning the tiles on a backing plate in a noncontact array.